Wednesday, September 03, 2008

Biocontrol Insect Exacerbates Invasive Weed -- Introduced flies create complex interactions that increase impact of invasive plant

Wow -
This is quite the mind-twister in terms of how humans are just one small cog in the wheel...

Humans introduce and spread damaging invasive knapweeds through contaminated agricultural seed supplies as well as through all the soil disturbances we create (via overgrazing, off road vehicles, etc.). Then we introduce an insect to control them - a natural predator of the weed from Europe. But unexpectedly, the 'biocontrol' agent insect ends up becoming a food source for native rodents, resulting in an explosion of their numbers. Of course, the main food source of these rodents is NATIVE plant seeds. The result is lower numbers of native plants, which creates more space for more knapweed to invade... Remember, this is a weed that causes ranchers tens if not hundreds of millions of dollars per year - not only in control costs, but also in lost forage, reduced livestock carrying capacity, and thus reduced land/real estate values.

Again and again, our unsustainable behavior ends up coming back to bite us in the ass. This is why the best solution to environmental problems like weed invasions is to prevent the human impacts that cause them from happening in the first place. The details of the study are below...

- Jon Gelbard

For immediate release: Wednesday, Sept. 3, 2008
Contact: Christine Buckley (202) 833-8773 x 211;
or Nadine Lymn (202) 833-8773 x 205;
Listen to the ESA podcast with Dean Pearson at

Biocontrol agents, such as insects, are often released outside of their
native ranges to control invasive plants. But scientists in Montana have
found that through complex community interactions among deer mice,
native plants and seeds, the presence of an introduced fly may
exacerbate the effects of the invasive plant it was meant to control.
The authors report their results in the September issue of the journal
Ecological Applications.

Spotted knapweed, a flowering plant native to Eurasia, was first
discovered in the United States in the late 1800s. This broad-leaved
plant has an advantage over native plants because its natural enemies,
including insects such as European gallflies, do not naturally exist in
North America. Thought to have hitched a ride with hauls of alfalfa,
knapweed is now widespread in western North America and has become a
serious problem in the U.S. across Washington, Idaho, Wyoming and
Montana and in Canada across Alberta and British Columbia.

As early as 1971, U.S. scientists began releasing gallflies in an effort
to reduce populations of the invasive weed. Like all biocontrol agents,
the gallflies were selected because of their specificity to their host
plant, leaving little risk of direct harm to other plants.

Adult flies lay their eggs in the weed's flowers, and after the larvae
hatch they induce the plant to grow tissue around the insect, encasing
it and isolating it from the rest of the plant.

"The woody galls wall off the fly larvae from within flower head," says
Dean Pearson, lead author on the study and a research ecologist with the
U.S.D.A. Forest Service's Rocky Mountain Research station. "The larvae
then overwinter in the seed heads for about nine months. When the plant
devotes all that extra energy to producing these galls, it has less
energy to produce seeds."

Scientists and managers expected that this seed deficiency would lead to
limited knapweed population growth. An unanticipated side effect,
however, involves the flies' furry neighbors. At the foot of the
Sapphire Mountains in western Montana, omnivorous deer mice, whose diet
usually consists of native seeds and insects, have also begun to prey on
the introduced gallflies.

"These mice are generalists and very effective at exploiting a new
resource," says Pearson. "They can tell which seed heads have the most
larvae inside them, and that makes them very efficient." Pearson says
that an average mouse can process 1200 larvae in one night. "A super
mouse could go through a whole lot more than that," he adds.

At Pearson's grassland study site, spotted knapweed makes up more than
half of the plant ground cover. The abundance of knapweed leads to lots
of gallfly larvae, which serve as a food subsidy for the mice. Pearson
and his coauthor, Ragan Callaway of the University of Montana, found
that this extra nourishment bolsters mouse population size, increasing
the numbers of hungry mice feeding on their original source of food: the
seeds of native plants. As mouse consumption of native plant seeds
increases, fewer native plants survive past the seed stage.

Pearson says that this exacerbation of the invasive species' impact has
a lot to do with the effectiveness of the fly at controlling the

"If the biocontrol agent is really effective, then it will eventually
eat itself out of house and home, and the community interactions become
less of an issue," Pearson says. He points out that even if the fly
decimates 80 percent of the knapweed population, the 20 percent of seeds
that are left to germinate are often enough to outcompete native plants.

The authors make the case that although biocontrol agents are carefully
selected for specificity to their host plants, these restrictions do not
prevent them from drastically altering the community food web, which can
have far-reaching repercussions. Pearson hopes that a better
understanding of food web ecology will lead to more effective tools for
invasion control.

"Everything's interconnected," says Pearson. "We need to understand the
ecology. If we can understand these complexities, we can attempt to
minimize the side effects and maximize the effectiveness of our tools."

To listen to a podcast of Pearson speaking about this paper in ESA's
Field Talk podcast series, please visit Pearson's
podcast is titled "Biocontrol Insects and the Mammals Who Love Them."

The Ecological Society of America is the world's largest professional
organization of ecologists, representing 10,000 scientists in the United
States and around the globe. Since its founding in 1915, ESA has
promoted the responsible application of ecological principles to the
solution of environmental problems through ESA reports, journals,
research, and expert testimony to Congress. ESA publishes four journals
and convenes an annual scientific conference. Visit the ESA website at

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